Fluid scrubbers are generally used to remove pollutants found in contaminated gases, such as waste gases from industrial operations. Pollutants that maybe removed by fluid scrubbers include fumes, gases, particulate, and oil mists that might be carried in waste gas streams. Within fluid scrubbers, pollutants are transferred from a gas stream and sequestered within a scrubbing liquid by mechanisms that include inertial impaction, reaction with a sorbent or reagent slurry, or absorption into a liquid solvent.
Scrubbing liquids are generally selected on the basis of costs and performance and may be solutions or slurries. In order to perform effectively, the scrubber liquid must have an affinity for target pollutants in order to create an adequate driving force for the pollutants to migrate from the gas phase and remain sequestered in the scrubber liquid. Depending on the nature of a particular target pollutant, suitable driving forces may include the solubility of the pollutant in the liquid phase or the affinity of a target pollutant for a particular reactant within the scrubber liquid. In the case of some pollutants that are in the form of particulate matter, thorough wetting of the surface of the particle through the mechanism of inertial impaction may be sufficient to allow capture and sequestration within the scrubber liquid. Some fluid scrubbing systems are configured to produce salable product as a means to improve the cost-efficiency of the scrubbing process.
In most conventional fluid scrubber systems the scrubbing liquid is collected in a sump and recycled back to the gas-liquid contact zone of the scrubbing system by a pumping system. The sumps usually includes mixing devices to maintain the scrubbing liquid in a homogeneous state and in some cases, such as flue gas desulfurization applications, the sumps also serve as reactors within which chemical reactions between sequestered pollutants and reagents are allowed to continue.
For fluid scrubbers to perform continuously and efficiently it is important to control the values of parameters within the scrubbing liquid that affect performance. The parameters to be controlled are specific to each application and may include one or more items such as the percentages of dissolved solids and/or suspended solids, reagents and other additives such as defoamers or wetting agents. In order to maintain the values of particular parameters within desirable ranges, scrubbing liquid is either periodically or continuously withdrawn from the sump (blow down) and an equal volume of fresh scrubbing liquid (make up) is added to the sump. Thus, the quality and total volume of scrubbing solution within the fluid scrubber may be maintained within desirable ranges.
While the methods used to manage blow down are specific to a particular scrubbing application the cost of managing blow down can have a significant impact on the overall costs of operation of fluid scrubbing system. Methods that might be employed to manage blow down include: direct discharge to a wastewater treatment plant; pre-treatment followed by direct discharge to surface water; treatment followed by recycle to the scrubbing system (e.g., removal, of suspended solids) or additional processing to recover salable material prior to disposal (e.g., recovering gypsum from flue gas desulfurization applications).
Other elements common to the design of conventional fluid scrubber systems are fans to move the gas stream through the scrubber and suitable means to separate entrained droplets of the scrubbing liquid from the gas phase before the gas is discharged from the scrubber. Positive pressure fans or induced draft fans located on the gas inlet or discharge sides of the scrubber, respectively, maybe used to move the gas phase through the scrubber. Various types of commercial demisting units are typically employed for separating liquid droplets from the gas phase. These units are usually located in proximity to the point, where the vapor exits the gas-liquid contact zone within the scrubbing system. The demisters generally capture entrained liquid droplets and return the collected droplets to the sump section either by gravity or a pump system.
In order lo perform effectively the fluid scrubber must include means for the scrubbing liquid and pollutant-laden gas stream to be brought into intimate contact so that target pollutants can be efficiently transferred from the gas phase into the liquid phase. Spray towers, packed towers and venturi devices are examples of conventional, types of fluid scrubbers that are in common use. As with most conventional fluid scrubbers, all three of these types of scrubbers rely on dispersing a discontinuous liquid phase into a continuous gas phase as the means of achieving the required intimate contact between the phases. However, distinct differences between the methods used to disperse the scrubbing liquid into the gas phase within each of these three types of conventional fluid scrubbers have a significant impact on the limitations of each type in relation to specific scrubbing applications.
Conventional spray tower fluid scrubbers typically use atomizing devices such as nozzles to disperse the scrubbing liquid as small, droplets into a spray chamber. Generally, smaller droplet sizes of scrubbing liquid improve the efficiency of spray tower fluid scrubbers by increasing the available surface area for intimate contact between the scrubbing liquid and gas phases. However, spray tower designs must strike a balance between the size of the atomized droplets and the energy requirements to first form very small droplets and then to separate such droplets from the flowing gas stream before the gas is discharged from the scrubber. Spray tower fluid scrubbers usually require significant headspace to accommodate the height of the tower. Among other factors, the height of the tower is generally a function of the time required to allow the liquid and gas phases to remain in contact in order for mass and heat transfer to reach equilibrium and for any possible chemical reactions to proceed to a desired degree of completion. Additionally, systems used for atomizing liquids within conventional spray tower scrubbers are prone to blockages and failure if suspended solids within the scrubber liquid build up and clog small passages of components such as nozzle orifices.
Conventional packed tower fluid scrubbers typically include spray nozzles or a weir at the top of a tower to uniformly distribute scrubbing liquid over packing within the tower. The packed tower type fluid scrubber relies on the extended surface area of the packing material to increase the contact area and contact time between the scrubbing liquid and gas by distributing the scrubbing liquid as a downward flowing film on the extended surface. Accordingly, packed tower type fluid scrubbers generally require regular maintenance (to clean the contact surfaces) and occasionally become clogged with suspended particles such as precipitates or insoluble particulate that is transferred from the gas phase. Thus, packed tower fluid scrubbers are not suitable for scrubbing applications that utilize slurries for scrubbing liquids. An additional draw back to the packed tower type fluid scrubber is that a balance must be struck between the amount of void space in the packed tower and the restriction that the packing presents to the flow of gas in the gas-liquid contact zone. The void space creates a tortuous path that forces the gas into intimate contact with the liquid film flowing over the packing. Smaller void space increases the velocity of the gas over the liquid film and enhances turbulence, which improves the rates of mass and heat transfer from the gas to the liquid phase. Because a finite amount of void space must be employed and additional space is occupied by the mass of the packing, the height and overall volume of packed tower fluid scrubbers are generally greater than that of other types of fluid scrubbers with the exception of spray towers.
Conventional venturi scrubbers use turbulence created by high differential pressure between gases flowing at high velocity through a restricted volume (the venturi throat) and free-flowing scrubbing liquid to create and disperse extremely small droplets of scrubbing liquid within the gas phase. Venturi scrubbers are considered to be high energy devices that are suitable for collecting very small particles. While venturi scrubbers are usually more compact than spray tower or packed tower fluid scrubbers, the energy required for forcing the combined vapor and liquid phases through the restricted volume of the venturi is generally higher than the energy requirements for other types of conventional scrubbers. Also, because all of the scrubbing liquid passing through the venturi is broken down into extremely small droplets in a highly turbulent zone, the demisting section to recover and separate the droplets from the gas stream generally requires higher energy input than the demisters used for packed tower and spray tower fluid scrubbers. Further, in addition to the energy burden, the high energy released into the restricted area of the venturi accentuates the potential of corrosion within the venturi section by creating significant erosive forces.
Conventional fluid scrubbers have other drawbacks as well. For example, within all fluid scrubbers substances that are dissolved in the scrubbing liquid may precipitate due to solubility limitations. Precipitates in combination with any insoluble particles that are transferred to the liquid phase from the gas phase (captured particles) may include substances or mixtures of substances that settle out and/or form agglomerates that can partially or fully block passages within the fluid scrubbing equipment. Periodic cleaning of fluid scrubber systems is required to offset the deleterious effects of built-up deposits of precipitates, captured particles and agglomerates. Also, because the maximum percentage of suspended solids in scrubbing liquids are limited by inherent constraints in the design parameters for spray tower, packed tower and venturi types of fluid scrubbers, the required rates of blow down in these fluid scrubbers can add significantly to the operating costs. While increased rates of blow down favor lower percentages of total solids, for a given fluid scrubber application, operating costs will rise with the blow down rate due to increases in: 1) the volume of blow down to be managed; 2) the consumption of fresh scrubbing liquid (make up) and; 3) the amounts of any reagents or other additives that have to be added to scrubbing liquid to overcome losses within the blow down.
Like most fluid scrubbers, submerged gas reactors/evaporators (hereinafter “submerged gas reactors”) generally mix liquids and gasses. However, unlike most conventional fluid scrubbers, within submerged gas reactors a discontinuous gas phase is dispersed within a continuous liquid phase. Submerged gas reactors are used in many industries to perform chemical reaction, processes with respect to various constituents. U.S. Pat. No. 5,342,482, which is hereby incorporated by reference, discloses a common type of submerged gas reactor in which combustion gas is generated and delivered though an inlet pipe to a dispersal unit submerged within the liquid to be reacted. The dispersal unit includes a number of spaced-apart gas delivery pipes extending radially outward from the inlet pipe, each of the gas delivery pipes having small holes spaced apart at various locations on the surface of the gas delivery pipe to disperse the combustion gas as small bubbles as uniformly as practical across the cross-sectional area of the liquid held within a processing vessel. According to current understanding within the prior art, this design provides desirable intimate contact between the liquid and the combustion gas over a large interfacial surface area while also promoting thorough agitation of the liquid within the processing vessel.
The design features of submerged gas reactors offset many of the drawbacks of conventional fluid scrubbers. For example, by dispersing the gas into a continuous liquid phase problems associated with removing entrained liquid droplets from the gas stream are greatly reduced compared to spray tower, packed tower and venturi type fluid scrubbers. Likewise, because submerged gas reactors do not rely on extended surfaces with critical void space requirements as exist in packed tower fluid scrubber designs, the potential to foul extended surfaces and block void space is eliminated. Also, because the gas phase flowing through a submerged gas reactor is dispersed as a discontinuous phase within a continuous liquid phase, for a given ratio of gas to liquid at a set pressure, the required volume of the gas-liquid contact zone is the minimum possible and generally a much smaller volume than that required in conventional spray tower and packed tower fluid scrubbers, thus favoring more compact designs compared to spray tower and packed tower scrubbers. Additionally, because submerged gas reactors do not rely on dispersing fine droplets of liquid into a continuous gas phase, demisters employed for separating entrained liquid droplets from the gas phase within submerged gas reactors typically consume significantly less energy than the demister sections required for venturi fluid scrubbers and somewhat less energy than demisters used in spray tower and packed tower fluid scrubbers. Further, because dispersing the gas into the liquid phase creates significant mixing action within the reaction vessel of submerged gas reactors the tendencies for particles to settle out of suspension and cause blockages are less than those within spray tower, packed tower and venturi type fluid scrubbers. Additionally, in combination with this mixing action, because a fixed volume of the liquid undergoing processing is always maintained within the submerged gas reactor vessel, the submerged gas reactor does not require a separate sump and mixer.
However, mitigation of crystal growth and settlement or agglomeration of suspended solids is dependent on the degree of mixing achieved within a particular submerged gas reactor, and not all submerged gas reactor designs provide adequate mixing to prevent settlement of solid particles and related blockages. For instance, settlement and agglomeration of solid particles can block critical passages within processing equipment such as the gas exit holes in the system described in U.S. Pat. No. 5,342,482. Liquid streams that cause deposits to form on surfaces and create blockages within process equipment are called fouling fluids.
Direct contact between hot gas and liquid undergoing processing within a submerged gas reactor provides excellent heat transfer efficiency. If the residence time of the gas within the liquid is adequate for the gas and liquid temperatures to reach equilibrium, a submerged gas processor operates at a very high level of overall energy efficiency. For example, when hot gas is dispersed in a liquid that is at a lower temperature than the gas and the resilience time is adequate to allow the gas and liquid temperatures to reach equilibrium at the adiabatic saturation temperature for the system, all of the available driving forces to affect mass and heat transfer, and allow chemical and physical changes to proceed to equilibrium stages, will have been consumed within the process. The minimum residence time to attain equilibrium of gas and liquid temperatures within the evaporation, reaction or combined reaction/evaporation zone of a submerged gas reactor is a function of factors that include, but are not limited to, the temperature differential between the hot gas and liquid, the properties of the gas and liquid phase components, the properties of the resultant gas-liquid mixture, the net heat absorbed or released through any chemical reactions and the extent of interfacial surface area generated as the hot gas is dispersed into the liquid.
Given a fixed set of values for temperature differential, properties of the gas and the liquid components, properties of the gas-liquid mixture, heats of reaction and the extent of the interfacial surface area, the residence time of the gas is a function of factors that include the difference in specific gravity between the gas and liquid or buoyancy factor, and other forces that affect the vertical rate of rise of the gas through the liquid phase including the viscosity and surface tension of the liquid. Additionally, the flow pattern of the liquid including any mixing action imparted to the liquid such as that created by the means chosen to disperse the gas within the liquid affect the rate of gas disengagement from the liquid.
Submerged gas reactors may be built in various configurations. One common type of submerged gas reactor is the submerged combustion gas reactor that generally employs a pressurized burner mounted to a gas inlet tube that serves as both a combustion chamber and as a conduit to direct the combustion gas to a dispersion system located beneath the surface of liquid held within a reaction vessel. The pressurized burner may be fired by any combination of conventional liquid or gaseous fuels such as natural gas, oil or propane, any combination of non-conventional gaseous or liquid fuels such as biogas or residual oil, or any combination of conventional and non-conventional fuels.
Other types of submerged gas reactors include hot gas reactors where hot gas is either injected under pressure or drawn by an induced pressure drop through a dispersion system located beneath the surface of liquid held within a reaction vessel. While hot gas reactors may utilize combustion gas such as hot gas from the exhaust stacks of combustion processes, gases other than combustion gases or mixtures of combustion gases and other gases may be employed as desired to suit the needs of a particular reaction process. Thus, waste heat in the form of hot gas produced in reciprocating engines, turbines, boilers or flare stacks may be used within hot gas reactors. In other forms, hot gas reactors may be configured to utilize specific gases or mixtures of gases that are desirable for a particular process such as air, carbon, dioxide or nitrogen that are heated within heat exchangers prior to being injected into or drawn through the liquid contained within a reaction vessel.
Regardless of the type of submerged gas reactor or the source of the gas used within a reactor, in order for the process to continuously perform effectively, reliably and efficiently, the design of the submerged gas reactor must include provisions for efficient heat and mass transfer between gas and liquid phases, control of entrained liquid droplets within the exhaust gas, mitigating the formation of large crystals or agglomerates of particles and maintaining the mixture of solids and liquids within the submerged gas reaction vessel in a homogeneous state to prevent settling of suspended particles.